The theory is in place, and we have no shortage of ideas as to how we can physically implement a quantum computer. But what might we use them for if we did? There are many suggestions – some practical, some highly fanciful

Ultrasecure encoding

One quantum information technology is already up and running. Various small-scale quantum cryptographic systems for secure information transfer, typically using polarised photons as their qubits, have been implemented by labs and companies such as Toshiba, Hewlett Packard, IBM and Mitsubishi. In October 2007, a quantum cryptography system developed by Nicolas Gisin and his colleagues at the University of Geneva in Switzerland was used to transmit votes securely between the city’s central polling station and the counting office during the Swiss national elections. A similar trial system developed by the researchers’ company, ID Quantique, was used to transmit data securely during the 2010 Football World Cup in South Africa.

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The distance through which quantum states can be transmitted through fibre-optic cables is limited to tens of kilometres owing to random diffusion. One promising way to get around this is akin to error correction protocols devised for quantum computers&colon; to spread information over more than one qubit (see “Quantum information&colon; The promise“). But this might pose a security risk by giving more information for an eavesdropper to hack.

Transmission via air is an alternative. The world record in faithfully teleporting a qubit …